.TH std::reverse 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::reverse \- std::reverse

.SH Synopsis
   Defined in header <algorithm>
   template< class BidirIt >                                \fB(1)\fP (constexpr since C++20)
   void reverse( BidirIt first, BidirIt last );
   template< class ExecutionPolicy, class BidirIt >
   void reverse( ExecutionPolicy&& policy, BidirIt first,   \fB(2)\fP \fI(since C++17)\fP
   BidirIt last );

   1) Reverses the order of the elements in the range [first, last).
   Behaves as if applying std::iter_swap to every pair of iterators first + i and (last
   - i) - 1 for each integer i in [0, std::distance(first, last) / 2).
   2) Same as \fB(1)\fP, but executed according to policy.
   This overload participates in overload resolution only if

   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.        (until
                                                                             C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
                                                                             C++20)

   If
   *first is not Swappable
   \fI(until C++20)\fP
   BidirIt is not ValueSwappable
   \fI(since C++20)\fP, the behavior is undefined.

.SH Parameters

   first, last -  the range of elements to reverse
   policy      -  the execution policy to use. See execution policy for details.
.SH Type requirements
   -
   BidirIt must meet the requirements of LegacyBidirectionalIterator.

.SH Complexity

   Exactly std::distance(first, last) / 2 swaps.

.SH Exceptions

   The overload with a template parameter named ExecutionPolicy reports errors as
   follows:

     * If execution of a function invoked as part of the algorithm throws an exception
       and ExecutionPolicy is one of the standard policies, std::terminate is called.
       For any other ExecutionPolicy, the behavior is implementation-defined.
     * If the algorithm fails to allocate memory, std::bad_alloc is thrown.

.SH Possible implementation

   See also the implementations in libstdc++, libc++, and MSVC STL.

   template<class BidirIt>
   constexpr // since C++20
   void reverse(BidirIt first, BidirIt last)
   {
       using iter_cat = typename std::iterator_traits<BidirIt>::iterator_category;

       // Tag dispatch, e.g. calling reverse_impl(first, last, iter_cat()),
       // can be used in C++14 and earlier modes.
       if constexpr (std::is_base_of_v<std::random_access_iterator_tag, iter_cat>)
       {
           if (first == last)
               return;

           for (--last; first < last; (void)++first, --last)
               std::iter_swap(first, last);
       }
       else
           while (first != last && first != --last)
               std::iter_swap(first++, last);
   }

.SH Notes

   Implementations (e.g. MSVC STL) may enable vectorization when the iterator type
   satisfies LegacyContiguousIterator and swapping its value type calls neither
   non-trivial special member function nor ADL-found swap.

.SH Example


// Run this code

 #include <algorithm>
 #include <iostream>
 #include <iterator>
 #include <vector>

 void println(auto rem, auto const& v)
 {
     for (std::cout << rem; auto e : v)
         std::cout << e << ' ';
     std::cout << '\\n';
 }

 int main()
 {
     std::vector<int> v {1, 2, 3};
     std::reverse(v.begin(), v.end());
     println("after reverse, v = ", v);

     int a[] = {4, 5, 6, 7};
     std::reverse(std::begin(a), std::end(a));
     println("after reverse, a = ", a);
 }

.SH Output:

 after reverse, v = 3 2 1
 after reverse, a = 7 6 5 4

   Defect reports

   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.

      DR    Applied to           Behavior as published              Correct behavior
   LWG 223  C++98      std::swap was applied to each pair of     applies std::iter_swap
                       iterators                                 instead
   LWG 2039 C++98      std::iter_swap was also applied when i    not applied
                       equals std::distance(first, last) / 2

.SH See also

   reverse_copy    creates a copy of a range that is reversed
                   \fI(function template)\fP
   ranges::reverse reverses the order of elements in a range
   (C++20)         (niebloid)
